The present disclosure pertains to a pigment-based inkjet ink and uses thereof in inkjet ink sets and inkjet printing. More particularly, the present disclosure pertains to an ink that is blue in color and comprises a mixed pigment colorant.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. Inkjet printers are equipped with an ink set which, for full color printing, typically comprises a cyan, magenta and yellow ink (CMY). An ink set also commonly comprises a black ink (CMYK).
Color characteristics of pigment-containing inks play an important role in the quality of the printed ink jet image. Perceived color quality can be characterized using any one of several color-space systems such as CIELAB (CIE No. 15.2, Colorimetry, 2nd. Ed., Commission Internationale de l'Eclarage, Vienna, Austria, 1986) or Munsell (Munsell Book of Color, Munsell Color Co., Baltimore, Md., 1929), as is well known in the art. With respect to Munsell color space, a given color is defined using three terms, namely Hue, Value and Chroma. With respect to CIELAB color space, a color is defined using three terms L*, a* and b*. In this system L* defines the lightness of the color and it ranges from 0 (black) to 100 (white). The terms a* and b* together define the hue, where a* ranges from a negative number (green) to a positive number (red) and b* ranges from a negative number (blue) to a positive number (yellow). Additional terms such as hue angle) (h°) and chroma (C*) can also be used to describe a given color instead of a* and b*, where
                    h        o            =                        tan                      -            1                          ⁡                  (                                    b              *                                      a              *                                )                      ;    and            C      *        =                                        a                          *              2                                +                      b                          *              2                                          .      
To achieve full color images, ink jet printers typically employ a cyan (“C”), magenta (“M”) and yellow (“Y”) ink. These colors are known as subtractively-mixing primaries, as light is subtracted as it passes through the colorant. When these colors are mixed in pairs they form red, green and blue (so-called “secondaries”), and when all are mixed together they form black. Thus these three-color inks can be used to print the entire range of hues with good chromaticity and/or color saturation.
The range of colors that a given ink set can print is called the color gamut, and can be described in the CIELAB system by the volume in the L*a*b* color space that encompasses a palette of representative colors that can be printed by the ink set on a given substrate. While a CMY ink set can produce all possible color hues, the color gamut does not encompass all the colors that can be perceived by the human eye. Thus there is still a desire to improve the printed image quality and extend the range of colors that can be printed.
Copper phtalocyanine pigment blue is widely used as a cyan pigment colorant in ink jet printing. However, the hue angle of this pigment is slightly bluer than a true cyan color and becomes increasingly more blue as the saturation or optical density of the color increases. Consequently, mixing of cyan inks containing copper phtalocyanine pigment blue with yellow inks tends to reduce the available color gamut in the green region. The present disclosure provides cyan ink compositions with a true cyan color and increased color gamut by mixing Pigment Blue 15 with Pigment Green 7.